92 



INTRODUCTION TO CLASSIFICATION. 



In the Pig the placenta is an infinitely simpler structure. 

 No " decidua " is developed ; the elevations and depressions of 

 the unimpregnated uterus simply acquire a greater size and 

 vascularity during pregnancy, and cohere closely with the 

 chorionic villi, which do not become restricted to one spot, but 

 are developed from all parts of the chorion, except its poles, 

 and remain persistent in the broad zone thus formed through- 

 out foetal life. The cohesion of the foetal and maternal placentae, 

 however, is overcome by slight maceration or post-mortem 



Fig. 42. 



Fig. 42. Section of the Human Uterus and Placenta at the thirtieth week of pregnancy. 

 (After Ecker.) A, umbilical cord ; B, chorion ; C, the foetal villi separated by pro- 

 cesses of; D, cavernous decidua ; E, F, G, wall of the uterus. 



change ; and, at parturition, the foetal villi are simply drawn 

 out, like fingers from a glove, no vascular substance of the 

 mother being thrown off. 



The process by which the mucous membrane of the uterus 

 returns to its unimpregnated condition after parturition in the 

 pig has not been traced. 



The extreme cases of placentation exhibited by Man and 



